public abstract class AttachableElement extends PhysicsElement {
   protected AttachableElement (int id) {
      super(id);
   }
   public abstract void attachSpring (Spring s);
   public abstract void detachSpring (Spring s);
   public abstract void attachRubber (Rubber r);
   public abstract void detachRubber (Rubber r);
   public abstract Vector2D getPosition();
}
   